Nanodomain Structure Research Articles

Laminated Modulation of Tricritical Ferroelectrics Exhibiting Highly Enhanced Dielectric Permittivity and Temperature Stability

AbstractFerroelectric materials with large dielectric response and high temperature‐stability have found significant applications in advanced electronics and electrical power/storage equipment. The effective approaches explored up to now mainly focus on improving dielectric response by employing the phase instability caused by the ferroelectric transition. Nevertheless, one inherent shortcoming is that the enhancement of dielectric permittivity is at the expense of the deterioration of its temperature stability. Here, a strategy that successfully achieves both enhanced dielectric response as well as excellent temperature reliability (with εr ≈ 2 × 104 from 30 to 85 °C) by designing a laminated structure of tricritical ferroelectrics (LTF) with successive Curie temperatures is proposed. Moreover, the improvement in dielectric performance triggers the temperature‐stable energy‐storage performance as well as electrocaloric property in LTF specimens. Further microstructure investigation and phase‐field modeling reveal that these remarkable properties of laminated layers originate from the successive occurrence of tricritical transition with a nanodomain structure in a wide temperature range. The findings may shed a new light on developing advanced ferroelectrics with high performance and thermal reliability.

Nanodomain structures in nonlinear crystal

Thermal properties of barium sodium niobate crystals were investigated by means of light scattering methods in the temperature range of 20–600°С. Three types of pictures, ellipses, rounds, stripes, have been observed on the screen in the far field depending on the temperature and polarization of the laser beam relatively to the crystallographic axes. The changes of the type of scattering are correlated with structural phase transitions at about 300 °C and 560 °C and with the evolution of the nano-domain structures at about 200 °C and 250 °C. Image analysis in the temperature region of 20–600 °C and comparison with the structural investigations below 300 °C allow to made conclusion that incommensurate phase in the temperature range of about 500–550 °C has the quasi-tetragonal symmetry.

Design of materials for solid oxide fuel cells cathodes and oxygen separation membranes based on fundamental studies of their oxygen mobility and surface reactivity

Design of materials for solid oxide fuel cells cathodes and oxygen separation membranes and studying their oxygen transport characteristics are important problems of modern hydrogen energy. In the current work, fundamentals of such materials design based on characterization of their oxygen mobility by oxygen isotope exchange with C18O2 and 18O2 in flow and closed reactors for samples of Ruddlesden – Popper-type oxides Ln2-xCaxNiO4+δ, perovskite-fluorite nanocomposites PrNi0.5Co0.5O3-δ – Ce0.9Y0.1O2-δ, etc. are presented. Fast oxygen transport was demonstrated for PNC – YDC (DO ~10-8 cm2/s at 700°C) nanocomposites due to domination of the fast diffusion channel involving oxygen of the fluorite phase with incorporated Pr cations and developed perovskite-fluorite interfaces. For LnCNO materials a high oxygen mobility (DO ~10-7 cm2/s at 700°C) provided by the cooperative mechanism of its migration was demonstrated. Depending on Ca dopant content and Ln cation nature, in some cases 1–2 additional channels of the slow diffusion appear due to decreasing the interstitial oxygen content and increasing the energy barrier for oxygen jumps due to cationic size effect. Optimized by the chemical composition and nanodomain structure materials of these types demonstrated a high performance as SOFC cathodes and functional layers in asymmetric supported oxygen separation membranes.

High-performance potassium-sodium niobate lead-free piezoelectric ceramics based on polymorphic phase boundary and crystallographic texture

Lead-free piezoelectric ceramics are urgently needed in the field of electromechanical conversion devices due to the restriction on the use of lead-based ceramics. In this study, the polymorphic phase boundary (PPB) were tuned by incorporating different concentration of (Bi0.5K0.5)HfO3 into the matrix (K0.5Na0.5)(Nb0.965Sb0.035)O3CaZrO3, and the <00l>c crystallographic texture was realized by templated grain growth method. The maximal d33 (∼550 pC/N) and kp (∼72%) were achieved in the <00l>c textured ceramics with composition around rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary. It is proposed that the enhanced piezoelectricity should be ascribed to several combined effects, which primarily contain the R-O-T phase boundary facilitating polarization rotation, the crystallographic orientation induced intrinsic piezoelectric anisotropy, electric-field-induced lattice distortion and phase transitions, and NaNbO3 seed-crystal-driven nanodomain structures. This work provides an effective solution to enhance piezoelectric properties by simultaneous tailoring polymorphic phase boundary and using crystallographic texture in potassium-sodium niobate based piezoelectric ceramics. We believe that the simple solution and design principle can also be applied to other piezoelectric ceramic systems, no matter lead-based or lead-free.

Outstanding piezoelectric properties, phase transitions and domain configurations of 0.963(K0.48Na0.52)(Nb0.955Sb0.045)O3−0.037(Bi0.50Na0.50)HfO3 ceramics

Requirements for substituting the current lead-based piezoelectric materials with lead-free ones are becoming increasingly strong from environmental concerns. In this article, we report piezoelectric properties, phase transitions and domain configurations of a dense 0.963(K0.48Na0.52)(Nb0.955Sb0.045)O3−0.037(Bi0.50Na0.50)HfO3 lead-free ceramic prepared through solid-state reaction by two-step sintering. It has a large piezoelectric coefficient d33 = 512 pC/N and a planner electromechanical coupling coefficient kp ≈ 0.54 at room temperature. Dielectric measurement indicates that rhombohedral-orthorhombic, orthorhombic-tetragonal, and tetragonal-cubic phase transitions upon heating process occur at −30 °C, 57 °C and 238 °C, respectively. X-ray diffraction analysis shows that the crystalline structure is dominantly of orthorhombic phase at room temperature. Domain configurations were investigated by acid etching technique. The poled ceramic shows domain patterns of long parallel stripes with hierarchical nanodomain structure existing in some broad stripes. The observed outstanding piezoelectric properties are ascribed to the rhombohedral-orthorhombic and orthorhombic-tetragonal phase transitions, and to the characteristic domain structure.

Piezoelectric performance, phase transitions, and domain structure of 0.96(K0.48Na0.52)(Nb0.96Sb0.04)O3−0.04(Bi0.50Na0.50)ZrO3 ceramics

Demand for replacing the current lead-based piezoelectric materials with some lead-free ones becomes increasingly strong from environmental concerns. In this article, we report the piezoelectric performance, the phase transitions, and the domain configurations of highly dense 0.96(K0.48Na0.52)(Nb0.96Sb0.04)O3−0.04(Bi0.50Na0.50)ZrO3 ceramics prepared by two step-sintering through solid-state reaction. This material has outstanding piezoelectric properties of piezoelectric coefficient d33 = 512 pC/N and electromechanical coupling coefficient kp ≈ 0.56 at room temperature. While d33 exhibits a broad peak and is greater than 430 pC/N between −30 °C and 70 °C, kp depends weakly on temperature below 50 °C but decreases considerably with further increasing the temperature. In terms of thermal aging, both d33 and kp remain stable from −50 °C to 240 °C. The degradation of kp quickly stabilizes in the first thermal cycle between −50 °C and 150 °C. Furthermore, the measurement of relative dielectric permittivity ε′ upon heating indicates that rhombohedral-orthorhombic, orthorhombic-tetragonal, and tetragonal-cubic phase transitions occur at TR-O ≈ −40 °C, TO-T ≈ 54 °C, and TC ≈ 265 °C, respectively. The X-ray diffraction analysis shows that the crystalline structure at room temperature is of orthorhombic-tetragonal phase coexistence. We also investigate the domain structure with an acid etching technique. The unpoled ceramic exhibits a complicated domain pattern consisting of irregularly shaped domains of long parallel stripes separated by 180° domain boundaries from neighboring domains. In contrast, upon poling, the domain pattern becomes simpler and takes the form of long parallel stripes of diverse widths, with a hierarchical nanodomain structure appearing inside some of the broader stripes. We consider that the superior piezoelectric properties and reasonable temperature stability are closely related to the rhombohedral-orthorhombic and orthorhombic-tetragonal phase transitions and to the characteristic domain structure.

Self-Organized Formation of Quasi-Regular Ferroelectric Nanodomain Structure on the Nonpolar Cuts by Grounded SPM Tip.

The understanding of self-organization processes at the micro- and nanoscale is of fundamental interest and is important to meet the great challenges in further miniaturization of electronic devices to the nanoscale. Here, we report self-organized quasi-regular nanodomain structure formation on the nonpolar cut of a ferroelectric lithium niobate single crystal. These structures were formed along the trajectory of grounded scanning probe microscope tip approaching or moving away from the freshly switched region. Detailed analysis of the formed structures revealed internal organization by the length of the needle-like domains, which ranged from uniform to quasi-periodic and even chaotic modes as a function of distance from the switched region. Comprehensive investigations and numerical simulations allowed to attribute explored phenomena to charge injection during the field application and further switching under the action of electric field induced by injected charges near the tip. Self-organization and quasi-periodicity were explained by the effective screening and long-range electrostatic interaction between the individual needle-like domains.

High piezoelectric performance and domain configurations of (K0.45Na0.55)0.98Li0.02Nb0.76Ta0.18Sb0.06O3 lead-free ceramics prepared by two-step sintering

Research for high-performance lead-free piezoelectric materials has become an urgent issue from the environmental concern. Very limited attempts on two-step sintering had been made so far. In this study, (K0.45Na0.55)0.98Li0.02Nb0.76Ta0.18Sb0.06O3 ceramics were prepared by both conventional sintering and two-step sintering. Piezoelectric properties, microstructure and domain structure were found to change significantly with sintering methods and sintering conditions. Two-step sintering was performed in the way that temperature is first quickly raised to 1180 °C, kept for 1 min, then immediately cooled down to 1120 °C and maintained for a desired time length. The effects of dwelling time on piezoelectric performance and microstructure as well as domain structure were investigated. High piezoelectric properties of d33 = 455 pC/N, kp = 0.54 and k33 = 0.67 were obtained in a ceramic prepared under the dwelling time of 20 h. This ceramic also possesses a very good piezoelectric thermal-ageing stability over −50 °C–150 °C. Further investigation reveals that this ceramic has a quite uniform grain-size distribution with the average grain size of about 12 μm in microstructure and shows domain patterns of simple parallel stripes with a hierarchical nanodomain structure appearing inside some of broad stripes. The observed excellent piezoelectric performance is considered to associate closely with the unique domain structure.

Micro-/nanodomains and their switching in a high Curie-temperature ferroelectric single crystal of Bi(Zn2/3Nb1/3)O3-PbTiO3

Domain structure plays a critical role in the functionality of ferroelectrics. In this work, the domain structure and local domain switching behaviors of a (001)C-oriented 0.38Bi(Zn2/3Nb1/3)O3-0.62PbTiO3 single crystal with high Curie temperature (~ 430 °C) and high tetragonality (c/a > 1.04, where c and a represent the lattice parameters of the tetragonal crystal), were studied by polarized light microscopy (PLM) and piezoresponse force microscopy (PFM). Local polar domains with typical sizes from several nanometers to micrometers were observed by PFM in the crystal. The local domain switching was realized by applying a voltage, indicating the ferroelectricity of the crystal. This work provides a better understanding of the micro-/nanodomain structure and related phenomena in ferroelectrics of complex perovskite structure.

Modulation of electrocaloric effect and nanodomain structure in Mn-doped Pb(In0.5Nb0.5)O3-PbTiO3 ceramics

The combination of electrocaloric (EC) and negative electrocaloric (NEC) effect in ferroelectrics is highly desirable for improving refrigerating efficiency and cooling capacity. However, the NEC effect is very rare because polarization decreases at elevated temperatures. In this work, the NEC effect in acceptor-doped ferroelectric ceramics with ABO3 perovskite structure was designed and simulated through a phenomenological model based on the Ginzburg-Landau-Devonshire theory. The coexistence of EC-NEC effects and enhanced wide range NEC effect were achieved in Mn-doped Pb(In0.5Nb0.5)O3-PbTiO3 (PINT) ceramic by adjusting defect dipoles. The virgin PINT ceramic only has positive EC effect (ΔT = 0.63 °C, under 40 kV/cm at 260 °C). However, coexistence of multiple EC and NEC was obtained in PINTMn4, which is valuable in improving the electrocaloric efficiency and cooling capacity. In PINTMn6, a single NEC effect was observed over a wide operating temperature range, from room temperature to 200 °C, and ΔT = − 0.62 °C was achieved at 173 °C, under an electric field of 40 kV/cm. In addition, similar island domain patterns were observed using piezoresponse force microscopy. The coexistence of positive and negative effects enhanced the NEC effect and a wide operating temperature range obtained in Mn-doped PINT bulk ceramic may aid in the development of high-efficiency solid-state cooling devices.

Self-Assembly of Two Unit Cells into a Nanodomain Structure Containing Five-Fold Symmetry.

Five-fold symmetry was forbidden for the periodic crystals until the discovery of the Al-Mn icosahedral quasicrystal. We report a kind of precipitated rod-shaped nanophase containing five-fold symmetry but not belonging to any crystals or quasicrystals discovered so far. These metastable nanodomain phases, which precipitated in Mg-6Zn alloy during isothermal aging at 200 °C, contain two separate unit cells in the 2D plane perpendicular to the five-fold axis but with periodic atom arrangement along the five-fold axis, that is, 72° rhombus structure and 72° equilateral hexagon structure. The self-assembly of two unit cells under some geometrical constraints into a nanodomain contains the 2D five-fold, C14, and C15 structures. This finding confirms the existence of solid matters in a special structure between the crystals and quasicrystals, and it is expected to provide a way to understand the atomic arrangement and stacking behavior in condensed matters.

Premartensite serving as an intermediary state between strain glass and martensite in ferromagnetic Ni-Fe-Mn-Ga

Previous investigations indicate that the two intrinsically different states, i.e., strain glass and premartensite in ferromagnetic Ni-Mn-Ga based system share some similarities in their transforming properties, such as nano-domain structure, elastic anomaly and temperature-induced transition to martensite. This indicates that the two states are possibly related with each other. However, the relationship between them is unclear so far. To explore this problem, we investigated the transforming behaviors of the ferromagnetic Ni55-xFexMn20Ga25 (x = 4–14) system. It was found that the transforming route of this system changes greatly with increasing Fe content. We also observed that the strain glass transforms into premartensitic state upon cooling, which is called spontaneous strain glass to premartensite transition. The local structure of strain glass is similar to that of premartensite. Moreover, a strain glass phase diagram of the Ni-Fe-Mn-Ga system was constructed, which reveals that the premartensite serves as an intermediary state between strain glass and martensite. With increasing Fe doping, the system changes from the martensite (long-range-ordered strain state with large spontaneous strain) into the premartensite (long-range-ordered strain state with small spontaneous strain) at first, and then the strain glass (short-range-ordered strain state with small local strain) is derived from premartensite by further doping Fe.

Ultrahigh Piezoelectric Properties in Textured (K,Na)NbO3 -Based Lead-Free Ceramics.

High-performance lead-free piezoelectric materials are in great demand for next-generation electronic devices to meet the requirement of environmentally sustainable society. Here, ultrahigh piezoelectric properties with piezoelectric coefficients (d33 ≈700 pC N-1 , d33 * ≈980 pm V-1 ) and planar electromechanical coupling factor (kp ≈76%) are achieved in highly textured (K,Na)NbO3 (KNN)-based ceramics. The excellent piezoelectric properties can be explained by the strong anisotropic feature, optimized engineered domain configuration in the textured ceramics, and facilitated polarization rotation induced by the intermediate phase. In addition, the nanodomain structures with decreased domain wall energy and increased domain wall mobility also contribute to the ultrahigh piezoelectric properties. This work not only demonstrates the tremendous potential of KNN-based ceramics to replace lead-based piezoelectrics but also provides a good strategy to design high-performance piezoelectrics by controlling appropriate phase and crystallographic orientation.

Demonstration of ultra-high recyclable energy densities in domain-engineered ferroelectric films

Dielectric capacitors have the highest charge/discharge speed among all electrical energy devices, but lag behind in energy density. Here we report dielectric ultracapacitors based on ferroelectric films of Ba(Zr0.2,Ti0.8)O3 which display high-energy densities (up to 166 J cm–3) and efficiencies (up to 96%). Different from a typical ferroelectric whose electric polarization is easily saturated, these Ba(Zr0.2,Ti0.8)O3 films display a much delayed saturation of the electric polarization, which increases continuously from nearly zero at remnant in a multipolar state, to a large value under the maximum electric field, leading to drastically improved recyclable energy densities. This is achieved by the creation of an adaptive nano-domain structure in these perovskite films via phase engineering and strain tuning. The lead-free Ba(Zr0.2,Ti0.8)O3 films also show excellent dielectric and energy storage performance over a broad frequency and temperature range. These findings may enable broader applications of dielectric capacitors in energy storage, conditioning, and conversion.

Molecular Tailoring of Poly(styrene-b-methyl methacrylate) Block Copolymer Toward Perpendicularly Oriented Nanodomains with Sub-10 nm Features.

We demonstrate a novel approach for fabricating vertically orientated, sub-10 nm, block copolymer (BCP) nanodomains on a substrate via molecular tailoring of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) BCP, one of the most widely used BCPs for nanopatterning. The idea is to incorporate a short middle block of self-attracting poly(methacrylic acid) (PMAA) between the PS and PMMA blocks, where the PMAA middle block promotes phase separation between PS and PMMA, while maintaining the domain orientation perpendicular to the substrate. The designed PS-b-PMAA-b-PMMA triblock copolymers, which were synthesized via well-controlled anionic polymerization, exhibited order-disorder transition temperatures higher than that of pristine PS-b-PMMA BCPs, indicating the promotion of phase separation by the middle PMAA block. For PS-b-PMAA-b-PMMA BCPs with total molecular weights of 21 and 18 kg/mol, the domain spacing corresponds to 19.3 and 16.7 nm, respectively, allowing us to fabricate sub-10 nm nanodomain structures. More importantly, it was demonstrated that the PMAA middle block, which has a higher surface energy than PS and PMMA, does not significantly alter lateral concentration fluctuations, which are responsible for phase-separation in the lateral direction. This enabled the vertical orientation of microdomains with sub-10 nm feature size on a PS-r-PMMA neutral surface without an additional neutral top layer. We anticipate that this approach provides an important platform for next-generation lithography and nanopatterning applications that require sub-10 nm features over large areas with simple process and reduced cost.

Observation of Exotic Domain Structures in Ferroelectric Nanodot Arrays Fabricated via a Universal Nanopatterning Approach.

We report a facile and cost-competitive nanopatterning route, using Ar ion beam etching through a monolayer polystyrene sphere (PS) array placed on a ferroelectric epitaxial thin film, to fabricate ordered ferroelectric nanodot arrays. Using this method, well-ordered BiFeO3 epitaxial nanodots, with tunable sizes from ∼100 to ∼900 nm in diameter, have been successfully synthesized. Interestingly, a plethora of exotic nanodomain structures, e.g., stripe domains, vortex and antivortex domains, and single domains, are observed in these nanodots. Moreover, this novel technique has been extended to produce Pb(Zr,Ti)O3 nanodots and multiferroic composite Co/BiFeO3 nanodots. These observations enable the creation of exotic domain structures and provide a wide range of application potentials for future nanoelectronic devices.

The structural origin of enhanced energy harvesting performance in piezoelectric perovskite

Energy harvesting, which can translate the wasted vibration energy into electric energy, is now a hot topic in the field of new energy, and the key point is to design high power piezoelectric ceramic according with the requirements of low-frequency vibration energy harvesting. In this study, high quality Co-modified 0.2Pb(Zn1/3Nb2/3)O3–0·8Pb(Zr0·50Ti0·50)O3 (PZN–PZT+Co) ceramics have been prepared by the two-stage method, and the energy harvesting characteristics were investigated. The results showed that the hierarchical nanodomain structure boosts the strong piezoelectric activity, leading to the high energy harvesting performance. The PZN–PZT+Co ceramic sintered at 1000°C exhibits an excellent d33×g33 value of 14080×10−15m2/N, which are much larger than that of commercial PZT-based ceramics. In the mode of the cantilever-type energy harvester, the output voltage and energy density of 33V, 4.4μW/mm3 were obtained at a low resonance frequency of 85Hz and acceleration of 10m/s2, showing potential application in piezoelectric energy harvester.

Morphological studies of small-molecule solar cells: nanostructural engineering via solvent vapor annealing treatments

Fine-tuning nano/micro-structural morphologies and their depth studies in small-molecule solar cells are key parameters in elevating device performances. In the current work, we focused on controlling the morphology of a small-molecule organic photovoltaic by treating its active layer film with solvent vapor annealing (SVA). For this project, the devices were made of all-small-molecule p-DTS(FBTTh2)2:PC71BM heterojunction films processed using different solvents for the vapor annealing. The different solvents resulted in different nano-domain packing structures and segregated phase separation as evidenced by detailed two-dimensional grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) and atomic force microscopy (AFM) analyses. From the data, we demonstrated a correlation between the nanostructural morphology of the active layer film and the performance of the device and found an SVA condition that produced a p-DTS(FBTTh2)2:PC71BM blend system yielding an improved device performance.

Investigation of domain walls in PPLN by confocal raman microscopy and PCA analysis

Confocal Raman microscopy (CRM) is a powerful tool for investigation of ferroelectric domains. Mechanical stresses and electric fields existed in the vicinity of neutral and charged domain walls modify frequency, intensity and width of spectral lines [1], thus allowing to visualize micro- and nanodomain structures both at the surface and in the bulk of the crystal [2,3]. Stresses and fields are naturally coupled in ferroelectrics due to inverse piezoelectric effect and hardly can be separated in Raman spectra. PCA is a powerful statistical method for analysis of large data matrix providing a set of orthogonal variables, called principal components (PCs).PCA is widely used for classification of experimental data, for example, in crystallization experiments, for detection of small amounts of components in solid mixtures etc. [4,5]. In Raman spectroscopy PCA was applied for analysis of phase transitions and provided critical pressure with good accuracy [6].In the present work we for the first time applied Principal Component Analysis (PCA) method for analysis of Raman spectra measured in periodically poled lithium niobate (PPLN). We found that principal components demonstrate different sensitivity to mechanical stresses and electric fields in the vicinity of the domain walls. This allowed us to separately visualize spatial distribution of fields and electric fields at the surface and in the bulk of PPLN.

Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary-Condition Engineering.

Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancement of modern electronic devices. Here, the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO3 ferroelectric films is reported. Using aberration-corrected scanning transmission electron microscopy, it is found that the spontaneous polarization in a 2 nm-thick ultrathin BiFeO3 film is abnormally increased up to ≈90-100 µC cm-2 in the out-of-plane direction and a peculiar rumpled nanodomain structure with very large variation in c/a ratios, which is analogous to morphotropic phase boundaries (MPBs), is formed. By a combination of density functional theory and phase-field calculations, it is shown that it is the unique single atomic Bi2 O3-x layer at the surface that leads to the enhanced polarization and appearance of the MPB-like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions.